Line voltage regulator utilizing line voltage responsive timing circuit to modulate duty cycle of controllable rectifier

ABSTRACT

An AC to DC converter regulates its output voltage with respect to line voltage changes by utilizing a line voltage responsive gating signal to control silicon controlled rectifiers in the output rectifier. The gating signals are generated by offsetting a voltage generated by a line voltage responsive timing circuit against a reference voltage.

United States Patent 91 La Duca 5] May 22, 1973 [54] LINE VOLTAGEREGULATOR 5 References Cited UTILIZING LINE VOLTAGE UNITED STATESPATENTS RESPONSIVE TIMING CIRCUIT TO MODULATE DUTY CYCLE 0F CONTROLLABLERECTIFIER 314121314 11/1968 3,417,312 12/1968 [75] Inventor Joseph Duca2,694,172 11/1954 Trousdale .321 18 x [73] Assignee: Bell TelephoneLaboratories, Incorporated, Murray Hill, NJ. Primary Examiner.l. D.Miller Assistant Examiner-H. Huberfeld [22] Flled' 1971 A tt0rney-W. L.Keefauver [21] Appl. No.: 201,636

[57] ABSTRACT 52 us. CI. ..32l/16, 321/18, 321/40, An AC to DC converterregulates its output voltage 323 22 SC with respect to line voltagechanges by utilizing a line 51 Int. Cl. ..H02m 7/22 voltage responsivegating Signal to control Silicon 58 Field of Search ..321 /l6, 18, 24,40; trolled rectifiers in the Output rectifier- The gating 307/252 N,252 P; 323/22 SC signals are generated by offsetting a voltage generatedby a line voltage responsive timing circuit against a reference voltage.

5 Claims, 2 Drawing Figures VOLTAGE SOURCE BACKGROUND OF THE INVENTIONThis invention relates to line voltage regulators. It is specificallyconcerned with the regulation of an AC to DC converter voltage output byphase control of the rectifying devices to compensate for changes in theinput voltage.

Typical circuit arrangements to stabilize the voltage output of an AC toDC converter with respect to variations in the input voltage utilizenonlinear magnetic devices such as saturating reactors and transformers.Most of these arrangements are operated in a ferroresonant mode, i.e., acapacitor is connected across the saturating reactor or transformer sothat. it resonates with the inductance at some particular voltage. Theresonant state is dependent on the magnitude of the applied voltagesince the inductance varies with the applied voltage. A small variationin the magnitude of the applied voltage changes the occurrence ofresonance of the circuit. Ferroresonant arrangements are excellent lineregulators. However, they are inefficient in operation because themagnetic devices must be saturated during each half cycle of operation.

Other line regulation arrangements may comprise the utilization of errordetectors and voltage regulation units which are separate and apart fromthe converter transformer. These arrangements, while more efficient thanthe ferroresonant arrangements, generally require a feedback circuitinterconnecting the output and input of the circuit. This arrangementgenerally includes a separate transformer in the feedback circuit toisolate the output and input sides of the converter.

It is, therefore, an object of the invention to stabilize the voltageoutput of an AC to DC converter with respect to variations in the inputvoltage.

It is another object of the invention to regulate the rectified outputvoltage of a converter transformer which operates in the linear regiononly without requiring a separate feedback circuit.

It is yet another object of the invention to maintain complete isolationbetween the input and output sides of a voltage regulated converter.

SUMMARY OF THE INVENTION According to the invention, the output voltageof an AC to DC converter is stabilized with respect to input voltagevariations by intermittently gating the rectified output voltage of theconverter. The output signal is gated in an inverse proportion tovariations in the magnitude of the input voltage. A gating controlsignal is derived by offsetting a reference voltage with anexponentially decaying voltage derived from the input voltage. Thereference voltage and the exponentially decaying voltage are bothderived from separate transformer windings on the secondary side of theconverter transformer. The gating control signal determines the firingpoint of the controlled rectifiers in the converter output rectifyingcircuit. since the exponentially decaying voltage is dependent on themagnitude of the line voltage, the duty cycle of the controlledrectifier is modified to compensate for changes in the line voltage.

BRIEF DESCRIPTION OF THE DRAWINGS Many additional objects and featuresof the invention will become apparent upon consideration of thefollowing detailed description of a specific AC to DC converterutilizing the principles of the invention. The following description isto be taken in conjunction with the attached drawing in which:

FIG. 1 is a schematic of an AC to DC converter embodying the principlesof the invention, and

FIG. 2 shows waveforms which illustrate the operation of the circuitshown in FIG. 1.

DETAILED DESCRIPTION The circuit shown in FIG. 1 is an AC to DCconverter which stabilizes the output voltage with respect to linevoltage variations by controlling the time that the output rectifierdevices conduct. The output rectifier devices are controlled by acontrol signal which is responsive to variations in the input voltage.

The AC to DC converter is driven by a basic energy source which, in theillustrative embodiment, comprises the AC square wave voltage source 15.The signal output of the voltage source 15 is shown by wave- 1 form A inFIG. 2 which depicts a square wave. While the driving signal for theconverter herein is shown to be a square wave, it is to be understoodthat the invention will operate equally well with signal inputs havingother periodic waveforms such as, for example, a sinusoidal waveform.

The output of the square wave voltage source 15 is connected to theinput terminals 11a and 11b of the input winding 11 of the convertertransformer 10. The converter transformer 10 includes an output winding12 whose two terminals 12a and 12b are connected to the siliconcontrolled rectifiers 30 and 20, respectively. The outputs of thesilicon controlled rectifiers 20 and 30 are connected to a common node45 to form a fullwave rectifier configuration. The node 45 is connectedto an output filter inductor 25 which, in turn, is connected to anoutput load 40. The opposite terminal of the load 40 is connected to thecenter tap 46 of the winding 12. The output filter also includes afiltering capacitor 26 which shunts the load 40 and a fiyback diode 24which connects the node 45 to the center tap 46. The operation offilters and fiyback diodes is wellknown and it is not believed necessaryto describe it in detail.

The switching of the silicon controlled rectifiers 20 and 30 iscontrolled by a gating signal arrangement which is responsive tovariations in the input line voltage appearing across the inputterminals 11a and 11b. The gating signal arrangement includes a timingcircuit 44 and a reference circuit 43. Each of these circuits isconnectedto the converter transformer 10. The reference circuit 43generates a reference voltage and the timing circuit 44 generates a timevarying voltage responsive to input voltage variations. The referencevoltage and the time varying voltage are combined to derive the gatingsignal utilized to turn on the silicon controlled rectifiers 20 and 30.

The reference circuit 43 is connected across the winding 13 of theconverter transformer 10. The reference circuit 43 includes a Diac diodel6 and a resistor 19 connected in series. A Diac is a bidirectionaldiode thyristor which may be utilized as a bidirectional volt agebreakdown device. A description of the characteristics of a Diac may befound in the Silicon Controlled Rectifier Manual, fourth edition by theSemiconductor Products Department of the General Electric Company. Thecircuit may utilize other bidirectional reference voltage devices suchas, back-to-back zener diodes or varistor devices in place of the Diacdiode 16.

The timing circuit 44 is connected across the winding 14 of theconverter transformer 10. The timing circuit includes a series connectedcapacitor 18 and resistor 17.

The Diac 16 and the resistor 17 are connected in series. The windings 13and 14 which energize the Diac 16 and the resistor 17, respectively, areoriented so that the voltage drop across the Diac 16 and the resistor 17oppose each other. The orientation of windings 13 and 14 is indicated bythe polarity dot notation shown in FIG. 1. The combined voltage acrossthe series connection of the Diac l6 and the resistor 17 is utilized asthe offset gating signal to control the silicon controlled rectifiers 20and 30. I

The principles of the operation of the invention may be readily acquiredby explaining a typical cycle of operation of the converter circuitdescribed above. The voltage waveforms in FIG. 2 illustrate the voltagesappearing at certain points in the circuit. For illustrative purposesassume that a square wave signal of the proper magnitude is applied bysource to the input winding 11 at the time T as shown in FIG. 2. Asshown by waveform A in FIG. 2, the polarity of this signal is positiveat time T This signal is applied to winding 11 so that the terminal 11Ais positive. Therefore, according to the winding orientation as shown bythe dot notation, the terminals 12a, 13a and 14a of the output windingsof the converter transformer 10 are also positive. At this instant thesilicon controlled rectifier 30 is biased in a forward direction but isnot conducting at the instant T A fixed reference voltage is generatedacross the Diac 16 which, at this instant, is positive to negative inthe direction from terminal 16a to 16b. This reference voltage is shownby waveform B in FIG. 2.

The timing circuit 44 comprising the capacitor 18 and the resistor 17 atthe time T develops an instantaneous voltage across the resistor 17. Atthe instant T there is no voltage across the capacitor 18 and all thevoltage across winding 14 appears across the resistor 17. The polarityof this voltage, shown by waveform C, is positive to negative in thedirection from terminal 170 to 17b. As shown by waveform C in FIG. 2,the voltage across resistor 17 opposes the reference voltage across theDiac 16.

The capacitor 18 begins to charge up and the voltage across the resistor17 decays exponentially. The opposite terminals of the series connectionof the Diac 16 and the resistor 17 are connected to the gate controlleads 31 and 21 of the silicon controlled rectifiers 30 and 20,respectively. The resultant offset voltage appearing across this seriesconnection is shown by waveform D in FIG. 2. As the voltage across thecapacitor 18 increases, the voltage across the resistor 17 decreases.Hence, the magnitude of the offset voltage applied to the gate terminal31 increases. The rate of this increase determines the moment in time atwhich the forward biased silicon controlled rectifier 30 begins 'toconduct. The operation of the converter during the subsequent half cyclein which the silicon controlled rectifier conducts is similar to thatdescribed above and hence is not described in detail.

The gating signal arrangement to control the switchingof the siliconcontrolled rectifiers 20 and 30 responds to changes in the input linevoltage to regulate the output voltage applied to the load 40. If themagnitude of the input voltage increases such as is shown at time T inFIG. 2, the initial voltage across the resistor 17 will be greater. Itwill take longer for the voltage across resistor 17 to decay and hencethe time at which the offset voltage increases to the triggering levelof the silicon controlled rectifiers 20 and 30 will be delayed. Hencethe voltage across the output load 40 will be reduced to its regulatedvalue. If the input voltage decreases in magnitude, such as shown attime T in FIG. 2, the initial voltage across the resistor 17 willdecrease below its normal value. Therefore, the offset voltage willincrease more rapidly and trigger the silicon controlled rectifiers 20and 30 in advance of their normal triggering point so that the voltageoutput across the load 40 may be maintained at its regulated value.

What is claimed is:

1. A line voltage regulated converter comprising a transformer includinga primary winding, a secondary winding, a reference winding and a timingwinding, a rectifier comprising first and second silicon controlledrectifiers connected to said secondary winding and including controlelectrodes, a reference voltage breakdown device coupled to saidreference winding, a timing network comprising a resistor and acapacitor connected in series and shunted across said timing winding,means to offset the voltage across said resistor against the voltageacross said reference voltage breakdown device and means to couple saidoffset voltage to the control electrodes of said first and secondsilicon controlled rectifiers.

2. A line voltage regulated converter as defined in claim 1 wherein saidmeans to offset includes said reference voltage breakdown device andsaid resistor connected in a series connection, the opposite terminalsof said series connection being connected the control electrodes of saidfirst and second silicon controlled rectifiers, respectively.

3. A converter circuit comprising input and output means, a transformercoupling said input and output means, controllable switching devicesincluded in said output means and having control signal inputs, means tocontrol said switching devices comprising:

reference voltage means comprising a first winding on said transformerand a voltage breakdown device shunting said first winding, a timingcircuit comprising a second winding on said transformer and an energystorage device and an energy dissipative device connected in series andshunting said second winding, means to offset the reference voltage dropacross said breakdown device with the voltage drop across said energydissipative device, and means to apply a voltage output of said means tooffset to said control signal inputs.

4. A converter circuit as defined in claim 3 wherein said energy storagedevice comprises a capacitor and said energy dissipative devicecomprises a resistor, said voltage breakdown device and said resistancebeing connected in series whereby the voltage output of said means tooffset is derived from the voltage drop thereacross.

5. A converter circuit as defined in claim 4 wherein said controllableswitching devices comprise two silicon controlled rectifiers connectedin a full wave rectifier configuration and the control signal inputsthereof are connected to opposite sides of the series connection of saidvoltage breakdown device, and said resistance, respectively.

at at :0:

1. A line voltage regulated converter comprising a transformer includinga primary winding, a secondary winding, a reference winding and a timingwinding, a rectifier comprising first and second silicon controlledrectifiers connected to said secondary winding and including controlelectrodes, a reference voltage breakdown device coupled to saidreference winding, a timing network comprising a resistor and acapacitor connected in series and shunted across said timing winding,means to offset the voltage across said resistor against the voltageacross said reference voltage breakdown device and means to couple saidoffset voltage to the control electrodes of said first and secondsilicon controlled rectifiers.
 2. A line voltage regulated converter asdefined in claim 1 wherein said means to offset includes said referencevoltage breakdown device and said resistor connected in a seriesconnection, the opposite terminals of said series connection beingconnected the control electrodes of said first and second siliconcontrolled rectifiers, respectively.
 3. A converter circuit comprisinginput and output means, a transformer coupling said input and outputmeans, controllable switching devices included in said output means andhaving control signal inputs, means to control said switching devicescomprising: reference voltage means comprising a first winding on saidtransformer and a voltage breakdown device shunting said first winding,a timing circuit comprising a second winding on said transformer and anenergy storage device and an energy dissipative device connected inseries and shunting said second winding, means to offset the referencevoltage drop across said breakdown device with the voltage drop acrosssaid energy dissipative device, and means to apply a voltage output ofsaid means to offset to said control signal inputs.
 4. A convertercircuit as defined in claim 3 wherein said energy storage devicecomprises a capacitor and said energy dissipative device comprises aresistor, said voltage breakdown device and said resistance beingconnected in series whereby the voltage output of said means to offsetis derived from the voltage drop thereacross.
 5. A converter circuit asdefined in claim 4 wherein said controllable switching devices comprisetwo silicon controlled rectifiers connected in a full wave rectifierconfiguration and the control signal inputs thereof are connected toopposite sides of the series connection of said voltage breakdowndevice, and said resistance, respectively.